Period· The columns of time

Tonian

1 Ga → 720 Ma

Rodinia comes apart, oxygen begins its second rise, and cells start eating other cells: the Neoproterozoic opens.

What the evidence shows

Rifts tear the supercontinent apart, and the stretching is what the Greek behind the name describes. The tearing has consequences beyond geography: fresh rock exposed to rain consumes carbon dioxide as it weathers, and the shallow seas along the new margins bury carbon in their sediments. Oxygen begins a second rise, a billion and a half years after the first, and it is a slow one: the deep ocean stays anoxic and iron-rich until late in the Neoproterozoic. In the shales the Grand Canyon now cuts through, microfossils between 780 and 740 million years old carry round holes bored clean through the wall, from 0.1 to 3.4 microns across and always one to a cell: the oldest direct evidence of predation on eukaryotes, one cell eating another from the outside in. The period also carries a contested claim about the origin of animals, structures in 890-million-year-old Canadian reefs that have been read as the tissue of keratose sponges, which would make them the oldest animal body fossils known.

Bounds and conventions

The base is a conventional round number; the top, by contrast, is tied to the onset of the Sturtian glaciation and moves as that dating improves. The sponge claim at 890 million years is contested, and nothing like a consensus has formed; molecular clocks put the first animals anywhere between 890 and 635 million years ago, which is to say across most of this period and the next.

Sources

1. Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
2. Turner (2021), Nature · Possible poriferan body fossils in early Neoproterozoic microbial reefs
3. Porter (2016), Proceedings of the Royal Society B · Tiny vampires in ancient seas: evidence for predation via perforation in fossils from the Chuar Group
4. Canfield et al. (2007), Science · Late-Neoproterozoic deep-ocean oxygenation and the rise of animal life
5. Pu et al. (2022), Science Advances · Emplacement of the Franklin large igneous province and initiation of the Sturtian Snowball Earth